Introduction

Seismic hazard assessment (SHA) of cities that have experienced exponential growth in the last 2–3 decades is important. The regional hazard zonation do not incorporate local effects leading to its infeasibility (impracticable) in land use development and planning, hazard mitigation and management, and structural engineering applications in site-specific terms. It is necessary to overcome these limitations, especially in the highly populated urban centers with unplanned urbanization practices. A quasi-deterministic or quasi-probabilistic approach employs a hybrid seismological, geological, geomorphological, and geo-technically guided framework in which all the potential hazard attributing features are considered with relative rankings in a logic tree, fuzzy set, or hierarchical concept (Nath et al. 2005). Seismic microzonation divides a region into sub-regions in which different safeguards must be applied to reduce, and prevent damages, loss of life, and societal disruptions, in case a large devastating earthquake strikes the region. It involves prediction of the hazard at different scales (say, from 1:50,000 to 1:25,000, 1:10,000, or 1:5,000 scales) (Nath et al. 2009). All geological, geotechnical, and seismological attributes are assessed to deliver the seismic microzonation in terms of a hazard index map. A microzonation project can be of different levels based on the mapping resolution, precision, data volume, and complexity of the problem (Bard et al. 1995).

The hazard during an earthquake is significantly controlled by geotechnical material properties. The site-specific attributes related to surface geologic conditions may affect the level of strong ground motions (Aki 1988; Field et al. 1992; Nath et al. 2000; Sitharam and Anbazhagan 2008). It is, therefore, important to estimate appropriate site-specific design ground motions for earthquake-resistant structural design and the hazard assessment. Since, multiple hazard components are involved, a robust decision support tool for hazard classification shall include every component and their likely contribution to the overall hazard. To that effect, seismic microzonation is carried out taking into account multiple criteria that accounts for several factors such as site response, shear-wave velocity, seismotectonics, landslide, geomorphological features, liquefaction assessment besides the strong ground motions (Sitharam and Anbazhagan 2008; Pal et al. 2008; Nath et al. 2008). After 2001 Bhuj earthquake, several studies related to the seismic hazard were carried out in different parts of the Gujarat (Singh et al. 2003; Iyengar and Raghukanth 2004; Yadav et al. 2008; Chopra et al. 2010, 2012, 2013). The city of Ahmedabad has experienced an intensity of VII on MSK-64 scale during 2001 Bhuj earthquake (Pande and Kayal 2003) and for which Iyengar and Raghukanth (2004) has estimated PGA of 0.15 g. Singh et al. (2003) and Chopra et al. (2010) have also estimated the strong ground motions due to 2001 Bhuj earthquake at various sites in Gujarat. Yadav et al. (2008) studied the probability of earthquake occurrence in three regions of Gujarat based on stochastic models at different time intervals. Chopra et al. (2012, 2013) carried out the deterministic seismic hazard assessment and estimated the earthquake strong ground motions for the entire Gujarat region at bedrock level and surface using stochastic finite-fault modeling.

The 2001 Mw 7.7 Bhuj earthquake was one of the most destructive earthquakes in India that killed around 14,000 people (EERI 2002). The damage was spread over a radius of 400 km including major cities of Ahmedabad, Bhavnagar, and Surat located at distances of more than 250 km. In these cities, many multistory buildings were collapsed. The site amplification is attributed to be one of the contributory factors for damages at far off distances. In Ahmedabad alone, 130 reinforced concrete frame buildings were either collapsed or seriously damaged (Goel 2001; EERI 2002; Murty et al. 2002). The Ahmedabad City is mushroomed on either side of the Sabarmati River, which flows from NE to SW. The area is characterized by more or less flat land topography with the major contours of 60, 40, and 20 m occupied by the sediments of Sabarmati River and Aeolian sands which were subsequently oxidized and pedogenized and the Aeolian sand imparts low height undulating topography. Numerous gullies and ravines are observed along Sabarmati bank on the northern side, while the depositional terraces were reported by GSI on the southern bank.

The study area lies in zone III (BIS 2002) where damaging earthquakes of magnitude up to 6 are expected. There is distance of over 240 km from Kachchh. To minimize the loss of life, property damage, social, and economic disruption caused by the earthquakes, it is essential that reliable estimates of seismic hazard should be available. The Ahmedabad is located on soft sediments of the Sabarmati river basin that are prone to amplification. This the India’s first heritage city declared by UNESCO.

In this work, geotechnical investigations are carried out in 23 boreholes, PS logging in 11 boreholes and multichannel analysis of surface wave at 54 sites in the Ahmedabad City. We have estimated the shear-wave velocity both by direct and indirect measurement. These investigations provided the distribution of shear-wave velocity in the city. This information is useful for estimating liquefaction potential and obtaining level of strong ground motions at the surface. The ground motion at the engineering bedrock (Vs 760 m/s) is estimated by PSHA analysis for 2% PE in 50 years, which corresponds to maximum credible earthquake (MCE). The pair-wise comparison of different attributes like shear-wave velocity, acceleration, liquefaction potential, and surface geology obtained from geotechnical and geophysical investigations is performed using AHP (Saaty 1987) to get an integrated seismic hazard map for Ahmedabad by giving suitable weightages and ranks to the attributes. Since the city is traversed by a major river that divides the city into two parts, the role of paleochannel in modifying the hazard is also investigated in the integrated hazard map of the Ahmedabad City.

Geological, geomorphological, and seismotectonic set up of the study area

Ahmedabad is one of the largest growing city of India founded in the 15th century with a population of around 6.2 million (Census 2011). It has an average elevation of 53 m from the mean sea level (MSL). It is situated on thick continental quaternary sequences of Sabarmati River basin (Tandon et al. 1997). The Ahmedabad City falls within the NNW-SSE–oriented Cambay rift basin bounded by east marginal and west marginal faults. These faults are dissected by many transverse faults (Wani and Kundu 1995). The quaternary deposits are about 300-m thick (Merh 1995). The alluvium mainly consists of alternate layers of fine to coarse grained sand, gravel, and clay. The alluvium is underlain by more than 3000-m thick tertiary sediments deposited over Deccan traps (Kaila et al. 1990). The quaternary succession of the Sabarmati basin consists of conglomerate, sandy, and silty soils and divided into four stratigraphic subdivisions viz. Waghpur Formation, Sabarmati Formation, Mahesana Formation, and Akhaj Formation (Tandon et al. 1997). The Waghpur Formation is characterized by well sorted fine buff sand, while the Sabarmati Formation is the youngest formation and consists of unconsolidated alluvium derived from Aravalli mountains (Sareen et al. 1993; Tandon et al. 1997).

Geomorphologically, the study area can be divided into residential upland, low land, dune, and interdunal regions with few water reservoirs or ponds (Fig. 1). The low land regions are further classified into flood plain, bad land, terrace, point bar, channel bar, and recent channel. The Sabarmati river flows through the middle of the Ahmedabad City (Fig. 1), which dries up in summer leaving only a small stream of water. Topographically, the study area is almost flat in nature except few small hills of Thaltej-Jodhpur Tekra. The average annual rainfall in the study area is 635 mm.

Fig. 1
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showing Borehole, PS logging, and MASW locations with geological and geomorphological setup of the study area

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The region is moderately active, seismically, and not much seismicity is observed in and around Ahmedabad City. A catalogue of earthquakes (Table 1) is prepared by ISR (2016-2017), and the historical seismicity map of the Ahmedabad City and surroundings is prepared (Fig. 2). Eight earthquakes of magnitude varying between 2.7 and 5.7 are observed within 80-km radius of Ahmedabad (Table 1). In the historical past, Ahmedabad experienced four earthquakes of magnitudes 4.6, 3.7, 5.7, and 3.7 in 1840, 1843, 1864, and 1897 respectively. Five other small shocks are cataloged. Moreover, a large earthquake in Kachchh region (M > 7) can affect high rise buildings in Ahmedabad City.

Table 1 Catalog of earthquakes during 1840 to 2016
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Fig. 2
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Epicenters of earthquakes M ≥ 1.5 occurring in the Ahmedabad and surrounding region during the period 1840 to 2016 (mark Ahmedabad in the map)

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Geotechnical investigations

A total of 23 boreholes (11 boreholes of 80 m, 7 boreholes of 40 m, and 5 boreholes of 35 m) were drilled at different locations in the city (Fig. 1). The boreholes are located at 2.5 km interval. During boring, soil samples (both disturbed and undisturbed) were collected for field and lab analysis to find out index properties of soil. The standard penetration tests were conducted, as per IS-code, at a depth interval of 3 m in every borehole. The SPT-N blow count and lithology were recorded during the sampling in the field and index properties of the disturbed and undisturbed samples of the soils, like grainsize analysis, Atterberg test, density, specific gravity was obtained and used for the seismic hazard assessment (Cavallaro et al. 2016). More than one thousand samples were collected (Dwivedi et al. 2017) and used for the seismic hazard assessment. The lithology of the study area generally varies from silty sand to clayey sand. The information pertaining to the depth of the water table is also collected during drilling. The ground water level of the study area is quite variable ranging from 3 to 50 m. These variations in water level may also be due to drilling of boreholes during different season. The SPT-N values measured in the field are generally low at shallower depths. The blow counts greater than 50 are generally encountered at depths of 6 to 9 m.

Geophysical investigations

For the estimation of VP and VS, the suspension PS logging test was carried out in 11 boreholes with depths ranging from 60 to 80 m. The suspension PS logging is relatively recent technology in geotechnical engineering and engineering geophysics. It is one of the best methods for the estimation of high resolution 1D VP and VS profiles in a single borehole at depths up to 80 m or even greater which are used for assessment of site characterization. It offers very high resolution (typically 1 m) data for resolving thin layers. The maximum measurable depth depends upon the maximum depth of open space available in a borehole. Generally, borehole collapse or mud filling at the bottom of the hole restricts depth of the Vs measurements. The suspension PS Logger probe is about 8.0-m long, comprises of a weight, source driver, source, filter tube, lower and upper geophone, head reducer, cable head, 4-conductor cable, winch, and logger/recorder (Sairam et al. 2018). Also, multichannel analysis of surface wave (MASW) is carried out at 54 sites covering the entire city. This technique utilizes the dispersive properties of Rayleigh waves for the estimation of a VS profile of the subsurface (Caruso et al. 2016). The entire process of MASW consists of three major steps, namely acquisition of multichannel seismic data, generation of the dispersion curve, and estimation of the VS profile. The recommendation of Park et al. (1999) is followed to obtain a laterally continuous 2D VS profile. A 48-channel engineering seismograph was used to acquire the data. The data were acquired using the standard CMP roll-along technique to achieve a continuous shot gather. A 30-kg weight is dropped on a metal plate to generate seismic waves. These waves were recorded on 24 vertical-component 4.5 Hz geophones/receivers planted at every 2-m interval along the profile, which responded to frequencies from 1 to 90 Hz in a 24-channel seismic shot gather. The acquisition parameters of the geometry are selected to optimize the imaging of subsurface layers down to more than 30 m depth. The signal-to-noise ratio of the processed data is more than 85 at each site. In the dispersion curve, the analyzed frequency was in the range 4–50 Hz. Each dispersion curve was individually inverted to obtain a VS profile by setting the initial model and performing a least-squares minimization (Xia et al. 1999) using SurfSeis (Kansas Geological Survey [KGS] 2010).

Attributes for hazard analysis

Site characterization

It is necessary to understand the subsurface lithology of on area to estimate the effect of earthquake at a site of interest. Many modern seismic codes of the developed countries like America, Europe, and Japan have produced abundant and geophysical data and have incorporated site effects in their codes (IBC 2009; UBC 97 1997; BSSC 2003; EC8 2011). The sites can be classified based on any one of the following three parameters, i.e., undrained shear strength (Su), standard penetration resistance (N-value), and shear wave velocity (Vs). The design spectrum determined 30-m average Vs values (Borcherdt 1994), 30-m average standard penetration resistance (N30), and undrained shear strength (Su), are also used for site classification. Both the Uniform Building Code (UBC 1997) and Eurocode 8 (EC8 2011) use VS30 to classify the sites, for earthquake engineering design. The Vs can directly be measured from field tests or can be estimated from the existing correlations between N-values and Vs (Hansancebi and Ulusay 2007). The VS30 estimation is useful for seismic zonation studies (Kockar et al. 2010). The site characterization of the Ahmedabad City is carried out based on average shear wave velocity to 30 m (VS30). The VS30 of the study area is estimated from the PS-logging and the MASW surveys and ranges from 163 to 380 m/s. The VS30 distribution in Ahmedabad is shown in Fig. 3. According to NEHRP (BSSC 2003), the study area comes under the site class D in which VS30 ranges from 180 to 360 m/s (Table 2).

Fig. 3
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Distribution of shear-wave velocity to 30 m (VS30) of Ahmedabad City

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Table 2 NEHRP classification of soils/rocks based on VS30
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Surface peak ground acceleration

Rout et al. (2019) prepared a probabilistic hazard map of Gujarat region at bedrock level (Vs 760 m/s) using an updates earthquake catalog from 1668 to 2016 by considering seven seismogenic zones in Gujarat based on the tectonic set-up and seismicity, which includes Cambay basin as a separate zone. The seismicity parameters were estimated for each seismogenic zone. The ground motion values were estimated at the B/C boundary (Vs 760 m/s) using the ground motion predication equation (GMPEs). The ground motion prediction equations (GMPE) are important in the seismic hazard analysis of any region. The accuracy of these prediction equations depends upon the data and methodology used to arrive at such relationships. These attenuation equations directly influence the hazard results. Generally, region-specific ground motion equations are preferred for the hazard estimation. For Gujarat region, few attenuation relations are developed using strong motion earthquake data. Mandal (2009) has developed a ground motion attenuation equation for the Kachchh region, Gujarat in terms of peak ground acceleration (PGA) by using aftershock data of 2001 Bhuj earthquake. NDMA (2011) has developed an attenuation equation for all the seven tectonic regions of India with different coefficients, and this equation is valid for site class A (VS30 > 1500 m/sec). The Eastern North America (ENA) is considered to be a stable continental region like Gujarat (Talwani 2014). Atkinson and Boore (2006) have developed a new ground motion equation for ENA using a stochastic finite-fault model for hard rock and soil types. The equations are valid for the hard rock site (VS30 ≥ 2000 m/sec) or NEHRP site class A. Rout et al. (2019) has used these GMPEs for PSHA with weightage of 40% to Mandal (2009), 40% to NDMA (2011), and 20% to Atkinson and Boore (2006), and the PGA has been calculated at different return periods.

The ground motions estimated at B/C boundary needs to be brought to surface level by applying amplification factors. The amplification factor for different sites are derived based on empirical relationship proposed by Choi and Stewart (2005) using the average shear wave velocity in the upper 30 m (VS30). By multiplying the amplification factor derived by Choi and Stewart (2005) with the ground motion value at B/C boundary level, the PGA is estimated at the surface level (Fig. 4). It is observed that the amplification is in the range 1.3–1.6 at Ahmedabad. It has been found that the PGA at surface varies between 108 and 144 cm/s2 for 2% PE in 50 years and between 52 and 65 cm/s2 for 10% PE in 50 years.

Fig. 4
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Distribution of surface peak ground acceleration in Ahmedabad City

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Factor of safety against liquefaction

Liquefaction hazard analysis was carried out based on SPT-N value, PGA, plasticity index, density, ground water level (GWL), and fine content (Fioravante et al. 2013; Cavallaro et al. 2018), using methodologies suggested by Seed and Idriss (1971), Youd et al. (2001) and Idriss and Boulanger (2006). The N values obtained from the standard penetration test are corrected to obtain the standardized (N1)60 (Robertson and Wride 1997). The measured SPT N values were also corrected for an overburden pressure in accordance with the recommendations made by NCEER ( 1997) and Kayen et al. (1992). The earthquake loading on the soil is expressed by the cyclic stress ratio (CSR). The simplified method suggested by Seed and Idriss (1971) is used to find out the CSR using following equation:

$$ \mathrm{CSR}=0.65\left(\frac{\alpha \mathrm{m} ax}{g}\right)\left(\frac{\sigma_{\mathrm{vo}}}{\sigma_{\mathrm{vo}}}\right)\mathrm{rd} $$
(1)

where 0.65 (αmax/g) = 65% of the peak cyclic shear stress; αmax = peak horizontal acceleration at the ground surface generated by the earthquake; g = acceleration due to gravity; σvo = total overburden stress; σvo’ = effective overburden stress; and rd = stress reduction coefficient. The strength of the soil (capacity of soil) to resist liquefaction is expressed in terms of cyclic resistance ratio (CRR). The liquefaction resistance of the soil depends on how close the initial state of the soil is to the failure.

$$ \mathrm{CRR}=\exp\ \left[\left(\frac{(N1)60\mathrm{CS}\kern1em }{14.1}\right)+{\left(\frac{N1\Big)60\mathrm{CS}\ }{126}\right)}^2-{\left(\frac{N1\Big)60\mathrm{CS}\ }{23.6}\right)}^3+{\left(\frac{N1\Big)60\mathrm{CS}\ }{25.4}\right)}^4-2.8\right] $$
(2)

Liquefaction resistance of the soil depends on how close the initial state of soil is to the state corresponding to the failure. The N value (N1)60 is further corrected for the fine content, based on the revised boundary curves as derived by Idriss and Boulanger (2006) for cohesion-less soils to get (N1)60CS, i.e., corrected N value for clean sand (Idriss and Boulanger 2006). The factor of safety (FOS) was calculated by the ratio of the CRR and the CSR and categorized as critical, moderate, low, and very low to classify the liquefaction probability within a region.

$$ \mathrm{FOS}=\frac{\mathrm{CSR}}{\mathrm{CRR}} $$
(3)

In the Fig. 5, it is found that the NW, NE, central, and the southern parts of the city have very low liquefaction potential whereas the eastern, western, and the SW parts show high to moderate liquefaction probability.

Fig. 5
figure 5

Distribution of factor of safety against liquefaction in Ahmedabad City

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Methodology

We tried to prepare an integrated seismic hazard map using various geophysical, geotechnical, and geological attributes following the analytical hierarchy process (AHP). The AHP technique is a multi-criteria method for scientific evaluation of the decision-making process proposed by Saaty (1987) and adopted for seismic hazard analysis. In this process, hierarchical structures are used to enumerate the relative significances of a given set of features on a ratio scale, which is based on the discretion of the researcher (Saaty 1987, 1988, 2003, 2008; Mohanty et al. 2007).

The hazard index is an integrated factor, and it depends on weights and ranks of the seismological and geological attributes. The attribute weights may be assigned based on their contribution to the seismic hazard. The rank can be assigned within an attribute against their values closer to hazards. Usually higher rank is assigned to an attribute, which is considered to play a significant role in deciding the overall seismic hazard of a region.

In this work, four attributes, namely, surface peak ground acceleration (PGA), shear-wave velocity to 30 m (VS30), liquefaction potential, and geology/geomorphology are taken for the preparation of integrated seismic hazard map for Ahmedabad region. Since the region has thick pile of sediments resting over the basement, we assigned highest weight to PGA followed by VS30, liquefaction, and geology/geomorphology (Table 3). The element matrices are prepared on a scale of 1–4. In this matrix, 1 indicates that both elements are equally important. Moreover, 4 implies that one is more important than the other element. Here, if one element is less important than other, it is indicated by the reciprocal, (i.e., 1/4). The pairwise comparison matrix is used to derive the normalized weight of each element. These calculated weights are used in deriving the weighted sums of rating of each area of the polygons of the mapped layers (Ganapathy 2011). The matrix prepared for the comparison is used for obtaining the normalized weights of each element. It is done by averaging the values of each row of the matrix. These weights also add up to 1 and can be used in deriving the weighted sums of rating for each region of polygons of the mapped layers. The different features can also be normalized between 0 and 1 to ensure that no layer can be influenced beyond its determined weight (Nath 2004). The normalization is carried out using the Eq. 4

$$ xi=\frac{Rj-R\min }{R\max -R\min } $$
(4)
Table 3 Assigned weights for Integration in GIS platform
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Here, Rj represents row rating; Rmin and Rmax show the minimum and the maximum rating of a particular layer (Nath 2004; Mohanty et al. 2007).

From this AHP, one spreadsheet is designed to calculate the weights of each layer. The pair-wise comparison with multiple criteria can be done. The calculations of the weights are done outside the GIS platform. The matrix is prepared for four attributes (Table 3) depending on their contribution to the hazard assessment. The PGA is assigned the highest weight. A similar type of approach for the seismic microzonation studies was successfully utilized by many researchers (Nath 2004; Mohanty et al. 2007; Ganapathy 2011). The weights for attributes PGA, VS30, liquefaction and geology/geomorphology are 0.4, 0.3, 0.2, and 0.1, respectively. The normalized rank of each thematic layer for integration in GIS platform is shown in Table 4. The integration will lead to an integrated seismic hazard map of the studied region.

Table 4 Normalized ranking of attributes
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Results and discussions

The lithological variations delineated from 23 boreholes suggest that the Ahmedabad City area made up of different layers of gravel, sand, and silty-clay (Dwivedi et al. 2017). The sediment layers encountered in the boreholes have shown distinct variations in their thickness and shapes from location to location. The index properties in the study area show variations in the characteristics of the soil. Irrespective of the location of borehole the top 3 m of the sedimentary sequences comprised of sandy silts to silty sand with clay. The boreholes 1, 2, 4, 6, 7, and 8 have relatively high concentration of finer sediments in the top 6 m, as they are located away from the main Sabarmati river, whereas, boreholes 3, 5, 9, 10, and 11 have higher concentration of sand as they fall near the Sabarmati River (Fig. 6). All the boreholes show presence of sandy sediments between 6 and 18 m, except at boreholes 4 and 6. At depths 18–30 m, all boreholes show silty—clayey sediments except at boreholes 1, 3, 5, and 10, which are located in palaeo-dunes and in the vicinity of Kankaria lake. At depths of 30–48 m, all boreholes show intercalations of silty sand and clay, except at boreholes 7, 8, and 10, which show sandy horizons, since they are located near to the main channel of the Sabarmati River. Similarly at depths 48–70 m, all boreholes show dominantly clayey horizons, except at boreholes 6, 7, and 10, which show silty sand horizons. Also, the boreholes 1 and 2 show coarsening of strata at depths of 57–70 m. The bottom-most strata at depths of 70–81 m of all the boreholes show mostly clayey horizons, except at borehole 8, which show the presence of silty sand. The reason of coarsening of horizons at deeper depths is indicative of the change in the hydrodynamic regime of the Sabarmati river. The boreholes closer to the main Sabarmati river show intercalations of sandy to silty-clayey horizons, which could be related to the change in flux of the Sabarmati river in past times owing to change in climatic conditions. On the western side of the river, a paleochannel was found ∼ 5.0 km away from present course (Goel 2001).

Fig. 6
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The variation of index properties of the soil in Ahmedabad region

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The water-table is generally deeper in the city except in boreholes 08, 07, and 20, where the water-table is at 3.5 m, 5.2 m, and 6.0 m, respectively. The specific gravity (~ 2.58–2.75) of the samples imply that the soil in the region is clayey and silty sand. The density (~ 1.3–2.15 gm/cc) implies that the soil is loamy. The void ratio found in the region (~ 0.36–0.80) generally support favorable conditions for local shear failure. The plasticity index (~ 5–35) indicates that the soil consist of clay with intermediate plasticity. The estimated values of cohesion (~ 0.1–0.9) and the angle of internal friction (~ 15–35) indicate that the soil has moderate to high shear strength. The areas like Vasna, Paldi, Satellite, Chandola, Isanpur, and Maninagar show very low N value of ~ 10–30 at shallow depth, indicating inadequate soil strength. These areas are covered by dunes and sand sheets.

The average shear-wave velocity (VS30) of the top 30 m is an important parameter for site characterization. The site classification scheme defined in NEHRP (1997) and Uniform Building code (UBC) (1997) are based on the VS30 values. The north-eastern, central, and south-western parts (Fig. 3) adjacent to the Sabarmati River show high VS30 (300–360 m/s) than the western and north-western parts (265–300 m/s). The lowest VS30 of about 205 m/s is observed in the western part of the Ahmedabad and the highest (380 m/s) is observed in the south-western part of the city. Over all, VS30 of the study area is in the range of 265 to 360 m/s. The western portion may comprise of more recent sediments as compared with eastern section. This region can be classified as D-type soil (stiff soil) according to NEHRP classification. The areas adjacent to the right bank of the Sabarmati river show lower VS30 as compared with the left bank shown in Fig. 3. This may be due to the presence number of ponds that were later reclaimed and a paleochannel of Sabarmati river (Fig. 1) that is present ~ 5 km west on the west-central side of the Sabarmati river (Goel 2001). The presence of unconsolidated sediments in top 6 m lowered the VS30 values in this region. This region may expect high amplification during large earthquake.

In this study, we have also estimated the level of ground motions expected at different sites using probabilistic approach. The PGA varies between 0.108 and 0.144 gals for 2% PE in 50 years at surface level. This is equivalent to PGA from a maximum credible earthquake (MCE). It is seen that PGA is maximum on the western side of Ahmedabad City where traces of a paleochannel is found. The high PGA is due to low VS30 and high amplification at these sites as local site response is included while estimating PGA at the surface. These PGA values along with index properties of soils determined by geotechnical investigations are helpful in estimating the liquefaction potential at various sites in the city using methodologies suggested by Seed and Idriss (1971), Youd et al. (2001) and Idriss and Boulanger (2006). It was found that southwestern part of the Ahmedabad City where the footprints of a paleochannel are present has high to moderate liquefaction potential.

The final seismic hazard assessment map is prepared by integrating different inputs obtained from geological, geotechnical, and geophysical investigations. For integration, we have considered PGA at surface, VS30, liquefaction potential and geology and geomorphology as attributes with weightages of 0.4, 0.3, 0.2, and 0.1 respectively (Table 3). The normalized ranks are provided to each layer of an attribute based on their values (Table 4). All the thematic layers are integrated on a GIS platform that results in an integrated seismic hazard map of Ahmedabad City. The integration was performed using UNION (SET theory). The entire city is categorized as having low, medium, or high seismic hazard (Fig. 7). The west-central part of Ahmedabad comprising of Satellite, Memnagar, and Jodhpur has highest seismic hazard (Fig. 7). This may be due to the fact the region has low VS30, has high amplification, and has high liquefaction potential. This area has a paleochannel of Sabarmati due to which the unconsolidated sediments are present at the top. The east-central portion comprising of Rakhiyal, Lal Darwaja, and Isanpur has moderate to high hazard. This region has lakes and ponds, most of which are reclaimed in recent years. During 2001 Bhuj earthquake (MW7.6), though the epicenter was 240 km from Ahmedabad, 130 reinforced concrete buildings were collapsed in Ahmedabad, and 752 people lost their lives (EERI 2002). The damaged area is marked in Fig. 7. The damaged area comprises of west-central and east-central regions of the city. The integrated seismic hazard map prepared by us show moderate to high hazard in those areas where damages were observed in 2001 earthquake. Though local site-effect played a role in damages, poor construction practices is also one of the reason for more damages (Sairam et al. 2018). The eastern most part of the city, comprising of Vastral and Naroda area, comes under the safe category. The soil is stiff in this area as compared to the soils of the western part of the city. In the northern part, the Chandkheda area and its surroundings are also categorized in the safe to moderate category.

Fig. 7
figure 7

Seismic microzonation Map of Ahmedabad City

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Conclusions

In this study, micro-level seismic hazard assessment of Ahmedabad City located in the western Indian state of Gujarat is carried out. The city declared as world heritage city by UNESCO. The city is situated in the middle of the moderately active Cambay rift. The city mushroomed across the banks of Sabarmati river that flows in the middle of the city. The city is located on 300-m thick quaternary sediments. Most part of the city is covered by flood plain of Sabarmati river. A paleochannel of Sabarmati river is also present in the central part of the city. The city is surrounded by east and west marginal faults of Cambay rift besides several transverse faults that crisscrossed the area. Not much seismicity is observed in this part of the rift barring few earthquakes of magnitude between 3.7 and 5.7. The region is vulnerable not only from a local large earthquake but also from a large earthquake in Kachchh region, which witnessed three large earthquakes in the last 200 years, namely 1819 Kachchh (MW7.8), 1956 Anjar (MW6.0), and 2001 Bhuj (MW7.6). Significant damages were observed in Ahmedabad during 1819 and 2001 earthquakes. A total of 23 boreholes were drilled varying in depths from 35 to 80 m spread all over the city. The index properties of the soil samples were determined in the field/lab in addition to SPT-N values for estimating the liquefaction potential. It was found that western and eastern parts of the city have high probability of liquefaction whereas northern and southern parts are safe. Geophysical investigations were also carried out at 65 sites (PS logging at 11 sites and MASW at 54 sites) to constrain the VS30 values. It was found that the entire city except a pocket in the southwestern part of the city is classified as D-type soil as per NEHRP. The VS30 values vary between 260 and 380 m/s. The VS30 values are lower at the sites located on the right bank of Sabarmati river and higher on the left bank. It seems that the presence of a paleochannel of Sabarmati river on the right bank is lowering the VS30 values as the region has unconsolidated sediments. The level of ground motions in terms of PGA expected in the region from local and regional earthquakes are also estimated using PSHA methodology for 2% PE in 50 years that corresponds to maximum credible earthquake. The PGA is first determined at engineering bedrock (VS 760 m/s) and then brought to the surface level using VS30 information at each site. Here too, it was observed that areas in and around paleochannel has more PGA as compared with other regions. An integrated hazard map considering the PGA, VS30, liquefaction, and geomorphology as attributes is prepared using AHP on a GIS platform by integrating thematic layers of all attributes. The city area is categorized as low, moderate, and high in terms of seismic hazard. It is observed that the west-central part of Ahmedabad coinciding with areas occupied by paleochannel has the highest hazard whereas northeastern, northwestern, and southern parts have low hazard. We inferred that presence of paleochannels, lakes, rivers, and reclaimed land controls the seismic hazard in a region. The integrated hazard map was compared with damages during 1819 Kachchh (MW7.8) and 2001 Bhuj (MW7.6) earthquakes. It was found that major damages reported from these two large earthquakes coincide with high hazard areas, validating the integrated hazard map. The hazard map can be used for the urban planning and infrastructural development.